Multicast communications for wireline users have been deployed in the Internet for at least the past 10 years. In such environments, a host joins a multicast group by informing a local multicast router that in turn contacts other multicast routers. A multicast tree is then created using typical multicast routing protocols. Along with the widespread deployment of wireless networks, the fast-improving capabilities of mobile devices, and an increasingly sophisticated mobile work force worldwide, content and service providers are increasingly interested in supporting multicast communications over wireless networks. Many new e-services can be made available if Multimedia Broadcast Multicast Services (MBMS) are available (e.g., distance education and entertainment services). In addition, important tactical information may be multicast to users (e.g., tanks, and planes) in emergency situations or battlefield environments. Supporting multicast features over wireless networks is an important and challenging goal, but several issues must be addressed before group applications can be deployed on a large scale over wireless networks.
In the interference-limited CDMA system, the downlink capacity is limited by the base station transmission power. The point to multipoint communication nature of MBMS requires higher base station transmission power than the unicast service for the similar application. There are two main issues that must be addressed in order to achieve the MBMS transmission efficiency: (1) lower target block error rate requirement than the unicast service for the same application, and (2) coverage over all MBMS group members. Specifically, because of intrinsic complexity associated with the multipoint-to-point feedback, the MBMS service has to be used in an unacknowledged mode. That is, no retransmission or ARQ is allowed to recover lost data blocks. The only available error control scheme is through channel coding. A channel without retransmission is much less tolerant to the block errors than a channel with retransmission. Therefore, the MBMS service must have a lower block error rate target than the unicast service for the same application. This translates into higher target signal-to-interference ratio (SIR) requirement and higher transmission power. Additionally, MBMS typically requires that all the MBMS group members in a cell can receive the service. Therefore, the required MBMS transmission power is determined based on the user who has the highest path loss to the base station. Statistically, maintaining reliable communication towards multiple users requires higher transmission power than towards a single user if the users are uniformly distributed within a cell. In addition, if power control is used, the transmission power has to be adapted to the user who suffers the highest instantaneous path loss to the base station.
As an example, FIGS. 1(a) and 1(b) depict a typical cellular communication site 100 in accordance with the state of the art. Specifically, the cell site 100 is composed of a base station 102 that functions as an antenna for distributing radio frequency signals to one or more cellular communication devices 104 (i.e. cellular phone, wireless PDA, lap top and the like). Information transmitted from the base station 102 is received from a larger wireless communication network (not shown for sake of simplicity). In typical MBMS, sufficient power must be generated by the base station so that signals transmitted therefrom can reach all communication devices 104 up to a cell boundary 106. The amount of power necessary to transmit these messages is shown graphically as a shaded region 108 inside the cell boundary 106. More specifically and as can be seen by FIG. 1(a) sufficient power must be transmitted by the base station 102 to completely fill the entire region 108 defined by the cell boundary 106. This type of transmission scenario represents the highest and therefore the most inefficient use of power because it does not take into consideration the fact that one or more users may not necessarily be at the cell boundary but at some point radially inward therefrom.
One particular solution to increasing power transmission efficiency is to dynamically alter the power setting of the base station 102. This scenario is further seen in FIG. 1(b). Specifically, FIG. 1(b) depicts the cell site 100 including base station 102 and one or more users 104 that are not located at the cell boundary 106. By monitoring the path loss of users 104 within cell site 100, it is possible to dynamically change the output power of the base station to encompass a smaller area. Such smaller area is depicted by the shaded region 110 radially inwards of cell boundary 106. While such a scenario does result in a reduced power output of the base station 102 it does not take into consideration the fact that one or more users 104 may be better served by one type of transmission scheme while other users at different locations within the cell boundary may be served by a different transmission scheme such that overall power output of the system can be reduced even further than that contemplated in either of the schemes depicted by FIGS. 1(a) and 1(b).